spongy brakes, bad booster?

[spf_lexus]

Ok I have a lot done to my braking setup, and didn't know if my larger fluid requirements are causing this or maybe aging part. I have supra TT fronts and larger LS400 1995 rears. I have oem rubber lines in rear and SS lines up front. My braking performance is good when I stomp it but for the pedal travel to be normal I need to pump the brake pedal one maybe twice before I actually push down hard, otherwise it's really hard to stop quickly.


I have no air in my lines and just flushed the fluid. Any ideas? Brake booster? Maybe master cylinder too small now for fluid needs? Hmmmm

 

[cjsupra90]

Forget the booster... Booster only prvides assist.. A bad booster would cause the pedal to be hard, not spongy...

Mastercylinder size can have an effect on this. Too small and there will be excessive peddle travel but it will seem that it doesn't take as much peddle effort to stop the car. To big will obviously be the opposite, stiff peddle and more peddle effort. If you have any problems with this at all, its gonna be the two big part as your mastercylinder is 1 1/16" bore where as the MKIV turbo supra mastercylinder is 1" bore.. So you having a larger bore will cause a stiffer peddle and requier more effort. Not a soft peddle.

My guess would be that you still have either air trapped somewhere, or you are getting what is known as pad kickback and this is causing the excessive peddle travel.. Just because your not getting air when you bleed does not mean that you dont have air trapped in it somewhere.. especially in the ABS pump module if you have ABS... I dont know for sure if yours requiers any special bleeding procedure but many ABS equiped sytems do requier special bleeding procedures for starter and also what method you used to bleed the brakes (vacuum, gravity, power, ect...) could all be issues as to whether or not you got all the air out...

Pad kickback could be a causing factor also. It the Disc faces are not perfectly parallel with eachother and with the caliper and also if the faces are not perfectly perpindicular with the axle, you will tend to get what is known as pad kick back. This is where the disc kicks (forces) the pad thus forcing the caliper pistons further back in the caliper. The then requiers you to have to move the peddle more to make up for this increased air gap before the pads bit.. This is more exagerated on cars that have to small of mastercylinders...

 

[spf_lexus]

Thanks for the info Cj,

I have bled mine a few times, but last time I still had the sponginess and took it to a specialist and it was flushed via machine, It was several quarts that fully circulated through the entire system, so i felt pretty confident it was done right.

The only other thing that i can think of is a leaky rear line, I know i wish I would have saw this but i think it's my issue. I never get low fuid but It "oozes" and it always wet around the screw type fitting where the solid line attaches to my rubber one. I feel if it can leak out... even slightly(ooze) then i'm sure th opposite is possible... inducing air but in very small amounts that are adding up. As for the swaps everything lined up find, I am still waiting to do the fronts at over 35,000 miles so far. I think ss lines with new crush washers?

 

[stevechumo]

I'd think there's some air, too. When I swapped with the 4 pots LS calipers for the front, I had to spend averagely 30 minutes on each to completely bleed it. You may check with the lines and make sure they're tightened and free of tangles. The rear rubber lines may be the problem when they get old and they flex/expand out of spec. You may change them to SS lines as the front.

 

[cribbj]

I had an issue with a spongy pedal after upgrading the calipers on my blue Supra. Bled them several times, even with the machine, but to no avail.

Finally brought the car to a shop run by an old dude who's forgotten more about brakes than I'll ever know, and as he was power bleeding them, he went around and tapped on each caliper a number of times with a rubber mallet. He claimed to have heard one caliper gurgle a bit after it was tapped, so he thought it may have had trapped air in it.

After this, I had a much more solid brake pedal.

 

[cjsupra90]

Thanks for the info Cj,

I have bled mine a few times, but last time I still had the sponginess and took it to a specialist and it was flushed via machine, It was several quarts that fully circulated through the entire system, so i felt pretty confident it was done right.

The only other thing that i can think of is a leaky rear line, I know i wish I would have saw this but i think it's my issue. I never get low fuid but It "oozes" and it always wet around the screw type fitting where the solid line attaches to my rubber one. I feel if it can leak out... even slightly(ooze) then i'm sure th opposite is possible... inducing air but in very small amounts that are adding up. As for the swaps everything lined up find, I am still waiting to do the fronts at over 35,000 miles so far. I think ss lines with new crush washers?

Yes, your oozing line could deffinitly be a problem... and that would be the first area i would fix before going any further... You might not even be sucking air back in, but even slights ooz like your describing will cause a loose of pressure.. The part that makes me even more think that that is a majority of your problem is cause its a rear.. As you may know, the brakes are splite into 2 circuits, one for the front brakes and one for the rear's.. The mastercylinder is splite into two circuits too and the back circuit of the mastercylinder handles the rear brakes. This circuit also helps to activate the front circuit. If there is any loose of pressure in the rear circuit, the front circuit will experiance a delay in building pressure due to the design of master cylinder piston assemblies. This delay is also in direct relation to a drop of the peddle to take up the space between the two piston and push rod assemblies in the mastercylinder..

One other thing to note here is a point that John brings up above about just becuase its was done with a machine or what not does not ensure that all the air is out... I lot of brake technitions at a lot of places do not realize that multi opposed piston caliper (like the supra 4pot units) are bad about trapping air in them.. IIRC, the Supra 4pots only have 1 bleed screw on the which makes them even worse with trying to get out all the air.. This is why most race caliper manufactures like Brembo, Alcon, Wilwood, AP Racing, ect... have two bleed screws (one on the inboard body half and one on the outboard body half). This makes them much less likely to trap air in them... Also IIRC, (and this may not be the case on your car) on the supra, they are mounted kinda high which starts to take them away from being perfectly vertical and this cause there to be an internal spot within the caliper that is higher then the bleed port thus causing air to be trapped in this area. A lot of times, the best thing to do is take the caliper off the spindle and place a block of something (wood or what ever) that is about the same thickness of the rotor inbetween the pads and hold the caliper in a way that ensures that the bleed screw and port are at the highest point they can be in relation to any area of the calipers internal fluid areas.. This ensures that there is no where for air to get trapped.. The Tapping trick like John said his guy did, does work quite well also...

 

[spf_lexus]

Great tips, I never gave any thought to the orientation of the bleed screw. I notice after 2-3 light pumps the 3rd will have perfect pedal travel while starting a decent stop.

I think once my leaky rear line is taken care of i will take all 4 corners off their spindles for both vertical bleeding and a drum solo with my rubber mallet. Thanks guys! learned a few things today.